Bipole power transmission systems are frequently used for transmitting Direct Current (DC) power between two or more terminals or converter stations of the power transmission system.
In its simplest form the DC power transmission system comprises one DC connection between a first converter acting as a rectifier and a second converter acting as an inverter.
A first converter station, which is provided at a first end of such a DC connection, will then comprise two converters connected in series between two pole lines via a neutral bus and the same configuration is typically provided at the opposite second end of the DC connection.
In this case the neutral bus on each side is typically connected to an electrode line which interconnects the neutral bus with a ground electrode for grounding purposes. It also makes the poles independent with regard to reliability.
It is at times necessary to disconnect one of the converters of such a converter station, for instance because there may be a fault or a converter needing maintenance. When this happens also the corresponding converter at the other end of the line has to be disconnected and refrained from being used. This will lead to the system in essence acting as an asymmetric monopole system. The disconnected converters and the pole lines they are connected to are thus all passive.
In some instances when there is an active and a passive pole line in the above described way, it is of interest to use a metallic return path via the passive pole line instead of or in addition to the ground return path via the ground electrode and the electrode line. There may exist a number of reasons for this, where one may be environmental concerns. In order to perform path changes DC breaking functionality is used in the the electrode line as well as in a current redirecting path used for connection to the passive pole.
The problem with such a change is that DC currents do not have any zero crossings. This has lead to the use of large and bulky DC breaker functionality being used in the electrode lines. In such a DC breaker, sometimes termed a metallic return transfer breaker, there is a switch connected in parallel with a resonance circuit formed by a reactor in series with a capacitor, which are in turn connected in parallel with a non linear resistor. In use the resonance circuit together with a negative resistance of an arc through the switch forms a current zero crossing enabling the opening of the DC switch. The non linear resistor in turn generates a counter voltage forcing the current through the electrode line over to the passive pole line.
However, the requirement of a resonance circuit and a non linear resistor makes the breaker complicated, bulky, maintenance intensive and also expensive.
The same problems are at hand also in the current redirecting path.
It is in view of what has been described above of interest to obtain a simpler arrangement that does not require circuit breaking functionality, but only a switch. If a resonance circuit and non linear resistor can be omitted then considerable simplification and savings are possible in the converter station.
The present invention addresses one or more of the issues mentioned above.